Aaron has announced a bzr plugin fault-line:

It works by looking at previous revisions where the file was changed,
and seeing what test files were changed at the same time. You can
specify the files, or it will autodetect them by looking at your working
tree.

For example:

$ bzr faultline
lib/lp/soyuz/test/test_archive.py
lib/lp/soyuz/scripts/tests/test_copypackage.py
lib/lp/soyuz/browser/tests/test_archive_webservice.py
lib/lp/code/windmill/tests/test_recipe_request_build.py